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A Case Study of Ozone Production, Nitrogen Oxides, and the Radical Budget in Mexico City

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A Case Study of Ozone Production, Nitrogen Oxides, and the Radical Budget in Mexico City Atmos. Chem. Phys., 9, 2499–2517, 2009 www.atmos-chem-phys.net/9/2499/2009/ Atmospheric © Author(s) 2009. This work is distributed under Chemistry the Creative Commons Attribution 3.0 License. and Physics A case study of ozone production, nitrogen oxides, and the radical budget in Mexico City E. C. Wood1, S. C. Herndon1, T. B. Onasch1, J. H. Kroll1, M. R. Canagaratna1, C. E. Kolb1, D. R. Worsnop1, J. A. Neuman2, R. Seila3, M. Zavala4, and W. B. Knighton5 1Aerodyne Research, Inc., Billerica, Massachusetts, USA 2Earth System Research Laboratory, NOAA, Boulder, Colorado, USA 3National Exposure Research Laboratory, Environmental Protection Agency, Research Triangle Park, North Carolina, USA 4Molina Center for Energy and the Environment, La Jolla, California, USA 5Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana, USA Received: 14 July 2008 – Published in Atmos. Chem. Phys. Discuss.: 19 August 2008 Revised: 25 March 2009 – Accepted: 31 March 2009 – Published: 7 April 2009 Abstract. Observations at a mountain-top site within the scale of one day. A new metric for ozone production effi- Mexico City basin are used to characterize ozone production ciency that relates the dilution-adjusted ozone mixing ratio and destruction, nitrogen oxide speciation and chemistry, and to cumulative OH exposure is proposed. the radical budget, with an emphasis on a stagnant air mass observed on one afternoon. The observations compare well with the results of recent photochemical models. An ozone 1 Introduction production rate of ∼50 ppbv/h was observed in a stagnant air mass during the afternoon of 12 March 2006, which is 1.1 Megacities and air pollution among the highest observed anywhere in the world. Approx- imately half of the ozone destruction was due to the oxida- Urban areas are often characterized by the presence of large tion of NO2. During this time period ozone production was emissions of pollutants, including volatile organic com- VOC-limited, deduced by a comparison of the radical pro- pounds (VOCs), nitrogen oxides (NOx≡NO+NO2), sulfur duction rates and the formation rate of NOx oxidation prod- dioxide (SO2), and particulate matter (PM). Many of these ucts (NOz). For [NOx]/[NOy] values between 0.2 and 0.8, compounds (e.g., benzene and diesel PM) are directly toxic gas-phase HNO3 typically accounted for less than 10% of − to humans (Khan, 2007; McCreanor et al., 2007; Yauk et NOz and accumulation-mode particulate nitrate (NO3(PM1)) al., 2008). These primary pollutants react in the atmosphere, accounted for 20%–70% of NOz, consistent with high ambi- forming secondary pollutants such as ozone, formaldehyde, ent NH3 concentrations. The fraction of NOz accounted for − nitric acid, and secondary PM. The spatial distribution of by the sum of HNO3(g) and NO3(PM1) decreased with pho- secondary pollutants and their direct effect on public health, tochemical processing. This decrease is apparent even when vegetation, and the Earth’s radiative balance can be quite dif- dry deposition of HNO3 is accounted for, and indicates that ferent than that of their precursor primary pollutants. Ozone HNO3 formation decreased relative to other NOx “sink” pro- (O3), in particular, plays a central role in tropospheric chem- cesses during the first 12 h of photochemistry and/or a sig- istry, as it is both a product and initiator of photochemistry nificant fraction of the nitrate was associated with the coarse as well as a potent greenhouse gas. Ozone is one of the US aerosol size mode. The ozone production efficiency of NOx EPA’s six criteria air pollutants subject to national ambient air on 11 and 12 March 2006 was approximately 7 on a time quality standards (NAAQS) created to protect public health. Numerous air basins across the world, spanning urban, sub- urban, and rural areas, have difficulty meeting relevant air Correspondence to: E. C. Wood quality standards for O3. Understanding the formation of O3 ([email protected]) and the transformations of its precursors (VOCs and NOx) is Published by Copernicus Publications on behalf of the European Geosciences Union. 2500 E. C. Wood et al.: Ozone, NOx, and radicals in Mexico City crucial for reducing air pollution on both urban and regional founding influence of transport. Secondary organic aerosol scales. Over the past several decades great effort has been formation at this site is analyzed and described in two related focused on characterizing air pollution and atmospheric pho- manuscripts (Wood et al., 2009; Herndon et al., 2008). The tochemistry in the US and Europe (Carslaw et al., 2001; Em- observations and inferences regarding tropospheric chem- merson et al., 2007; Kleinman, 2005; Nunnermacker et al., istry are compared to the predictions of several photochem- 1998; Ryerson et al., 2001; Tressol et al., 2008). However, ical models (Lei et al., 2007; Madronich, 2006; Tie et al., there are few studies of tropospheric photochemistry in de- 2007). Such observational-based characterizations of pho- veloping megacities – where much of the projected increases tochemistry, even if focused on a short period of time, are in population and pollutant emissions over the next several crucial for testing our understanding of the underlying pho- decades are expected to occur (Molina et al., 2004). tochemical processes that control secondary air pollution. During the 2006 MILAGRO (Megacity Initiative: Local And Global Research Observations) campaign, Mexico City 1.2 Urban photochemistry overview was used as a test case for developing megacities. Mexico City is currently the third most populous urban agglomer- Tropospheric ozone is produced by the oxidation of VOCs ation in the world, with a population of 19 million (UN, (volatile organic compounds) in the presence of nitrogen ox- ≡ 2008). Although the air quality in Mexico City has improved ides (NOx NO+NO2) and sunlight. Central to understand- greatly over the past 10 years (de Foy et al., 2008), it remains ing ozone production is the photostationary state formed be- highly polluted, with ozone concentrations frequently ex- tween NO, NO2, and O3 in sunlight: ceeding 100 ppbv. The motor vehicle fleet, consisting of over NO + O3 −→ NO2 + O2 (R1) 4 million cars and trucks, is older than the US fleet, and ap- 3 proximately 30% of light-duty gasoline vehicles do not have NO2 + hν(λ < 400 nm) −→ NO + O( P) (R2) functioning 3-way catalytic converters (CAM, 2008). The 3 + + −→ + latitude (19◦ 24◦ N) and altitude of the Mexico City plateau O( P) O2 M O3 M (R3) (2.2 km above sea level) leads to elevated actinic flux levels Net reaction: null throughout the year. The high altitude also tends to cause NO can also be oxidized to NO2 by compounds other than fuel-rich combustion in the vehicle fleet. O3, most importantly hydroperoxy radicals (HO2) and or- Inferences about tropospheric photochemistry based on ganic peroxy radicals (RO2, where “R” represents an organic measurements from stationary sites are often complicated group): by the fact that the observed changes in pollutant concentra- tions over time are a function of not just chemistry but also HO2 + NO −→ OH + NO2 (R4) meteorology and emissions patterns. Additionally, the air + −→ + masses observed usually contain a range of aged and fresh RO2 NO RO NO2 (R5a) pollutants, since primary emission sources of NO , VOCs, x During the day, most of the NO2 formed from R4 and R5a and PM continue to impact most stationary sites through- undergoes photolysis, leading to the creation of ozone (R2 out the day. Measurements on board instrumented aircraft and R3). When discussing ozone production quantitatively, are often better suited for tracking the evolution of an air it is useful to use the concept of Ox (“odd-oxygen”), which mass over time (Ryerson et al., 2001), though the first 3– in the troposphere is often defined as the sum of O3 and NO2 7 h of atmospheric oxidation following sunrise usually occur ([Ox]≡[O3]+[NO2]). NO2 is included because it acts as a in a shallow boundary layer near the urban surface where it reservoir of O3, formed by R1 or as a source of O3 (R4 and may be unfeasible to fly. Smog chamber studies have pro- R5a). The sum of the rates of R4 and R5a is therefore equated vided insight into numerous atmospheric processes, but are to the instantaneous production rate of Ox: challenged by wall interactions and the difficulty in realisti- X cally mimicking atmospheric parameters such as actinic flux, P(Ox) = k5ai[RiO2][NO] + k4[HO2][NO] (1) low pollutant concentrations, and total atmospheric composi- i tion. Some atmospheric processes have not been adequately Although the right-hand side of Eq. (1) is often defined characterized by laboratory studies, e.g. secondary organic as P(O3) (i.e., the instantaneous production rate of O3 rather aerosol formation (de Gouw et al., 2005; Volkamer et al., than Ox), by defining it as P(Ox) instead there is no need 2006). This emphasizes the importance in quantifying such to account for reactions of NO2 subsequent to its formation, processes using atmospheric measurements. and the expression holds true at all light levels. We note that In this paper, we present measurements from Pico de Tres during the day, typically over 95% of the NO2 will undergo Padres, a unique mountain-top stationary site that is situated photolysis to form O3 (the remainder reacts to form HNO3 within the Mexico City basin but is minimally impacted by or other compounds), and thus the terms “Ox production” nearby emissions. The air masses observed on 12 March and “ozone production” are often nearly equivalent. How- 2006 were stagnant in the afternoon and provided an excel- ever, in the presence of large concentrations of NO, pho- lent opportunity to study ozone chemistry without the con- tochemically formed Ox may appear mainly in the form of Atmos.
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